The overall objectives of this project are divided into three areas: 1) Studies relating to the localization, characterization and purification of the mineralocorticoid receptor (aldosterone-binding protein). 2) Studies relating to the mechanism and regulation of sodium transport in the urinary bladder of the toad. 3) Studies relating to the mechanism and regulation of urinary acidification by the urinary bladder of the toad. In the current year the studies were directed at the identification of proteins in the mucosal membrane by iodination of the mucosal surface of the toad bladder: In studies on the mineralocorticoid receptor in a dog kidney cell line, inhibitors of serine-residue proteases were found to inhibit steroid binding. BIBLIOGRAPHIC REFERENCES: Masters, B.R., D.D. Fanestil, and J. Yguerabide. The role of membrane fluidity on the permeability changes of ADH-stimulated toad urinary bladder. Biophys. J. 17:92a, 1977. Baker, M.E., D.A. Vaughn, and D.D. Fanestil. Effect of protease inhibitors and substrates on deoxycorticosterone binding to its receptor in dog MDCK kidney cells. Fed. Proc. 36-267, 1977.